In a wireless communication system that includes a base station (e.g., an evolved Node B) and a plurality of mobile terminals (e.g., a plurality of mobile phones or other mobile devices capable of wireless communication with the base station), the base station may be configured to control the transmit power of the mobile terminals. For example, the base station may employ a method to limit the transmit power of one or more of the mobile terminals for the purpose of improving an overall signal to interference and noise ratio (SINR) for the cell served by the base station. As an example, the base station may employ a closed loop power control scheme to improve the SINR. In a closed loop power control scheme, the base station determines the SINR in the cell and then transmits individual transmit power control (TPC) messages to the mobile stations based on the determined SINR. Each TPC message transmitted to a mobile terminal includes data (e.g., a value) that is used by the mobile terminal in setting its transmit power. Such data may be referred to as a TPC command. In some systems (e.g., in long term evolution (LTE) communication systems) the SINR determination is done continuously for every new radio frame. A combination of open loop and closed loop power control may be used in LTE, and closed loop around an open loop. For example, a mobile terminal may set an open loop power based on measurements on reference signals from the base station, and the TPC commands sent from the base station define an adjustment to the open loop power (i.e., an amount by which the mobile terminal should increase or decrease the power).
For a mobile terminal that has not transmitted data to the base station within some predetermined period of time (e.g., a mobile terminal that has been inactive for some time), the base station will not have an up-to-date measurement of the mobile terminal's signal quality. Accordingly, when such an inactive mobile terminal subsequently has data to transmit to the base station, the base station will not have a good way to determine the value at which to set the mobile terminal's transmit power (e.g., the TPC command) for the mobile terminal's initial transmissions. This may lead to the mobile terminal either (1) transmitting its data using more transmit power than necessary, thereby causing excessive interference to the cell or (2) transmitting its data using insufficient output power, thereby increasing the likelihood that a re-transmission will be needed.
The amount of time it takes for a base station to measure a mobile terminal's signal quality is on the order or 2-10 ms. Because the radio bitrate can be very high (e.g., up to 85 Mbps in an LTE uplink), the transmission time is in many cases very short, thus the amount of data transmitted by the mobile terminal during this 2-10 ms period may be a significant fraction of the mobile terminal's transmissions.
In LTE systems, there is a possibility to control the power of a mobile terminal when it is not active with TPC-physical downlink control channel (TPC-PDCCH), but there is no channel on which to measure signal quality. Uplink sounding can be turned on, but this is a very costly alternative for maintaining power control for inactive mobile terminals.